The use of liquid compositions for automatic home dishwashing offers several advantages over the more predominant powdered or granular forms. These advantages include greater ease of handling in dispensing and dosing, the substantial elimination of lump formation, "caking", and dust, and improved solubility.
However, satisfactory liquid autodish detergent compositions must meet certain requirements. First, the composition must be a substantially uniform mixture of ingredients to deliver the optimum combination of active components to the wash with each dose. In most current formulations, this requires that the liquid be shaken before each use to remix the components. A satisfactory product should also be substantially stable against physical separation and segregation of its active components or de-mixing. In addition, a high composition viscosity at a low shear rate contributes to physical stability of the liquid and protects against separation of the active components.
Physical stability can be achieved through the use of suspending or viscosifying systems to enhance the liquid rheological properties. Such systems typically maintain viscosity at low shear rate under the high ionic strength conditions present in a built liquid detergent. The agents producing these systems must also be chemically compatible with the other components of the formula, especially chlorine bleach or hypochlorite ion at the high pH where the ion is stable.
A further constraint is that the liquid dishwashing detergent must also be compatible with the dishwashing equipment presently available. Most current home dishwashing machines use detergent cups which have been designed to house powdered or granular solid detergent and deliver it to a specific wash cycle. The cups are not designed to contain low viscosity liquids. Consequently, any liquid for use as an automatic dishwashing composition or detergent must possess a sufficiently high viscosity to be effectively retained in the cup to avoid substantial leakage into the machine during cycles which precede the wash. Excessive leakage leads to under-dosing in the wash cycle and may negatively affect cleaning performance. Although high viscosity is desirable under storage conditions or while the material is in the detergent cup, the liquid must also be readily and conveniently dispensed from its container. Therefore, a liquid that undergoes a viscosity decrease under the influence of applied shear such that the decrease is reversible with time after the removal of shear, is preferable. This behavior is termed thixotropy and is desirable for liquid dishwashing detergents. Agitation of the liquid in the container, by squeezing or shaking, will supply sufficient shear strain to initiate shearthinning behavior and increased liquid flow for dispensing from the container. Optimum flow properties allow for easily pourable liquids or fluids which maintain sufficient viscosity at higher shear rates to prevent or minimize excessive spillage. The liquid must also quickly regain its structure or viscosity after dispensing so it does not undergo substantial leakage from the dispenser cup in the machine.
GB 2 164 350 describes a liquid automatic dishwashing product comprising a liquid phase which is water containing alkali metal tripolyphosphate, clay thickener, a chlorine bleach compound and a water-soluble polymeric carboxylic acid, for example, sodium polyacryate. GB 2 176 495 describes clay thickened liquids stabilised by polyvalent metal salts of long chain fatty acids, for example aluminium tristearate.
It has been discovered that a hypochlorite containing liquid automatic dishwashing detergent composition which includes a structuring system of a swellable clay, a water-soluble synthetic polymer, and a source of multivalent cations substantially minimizes the problems of the art. This combination also gives a positive effect on the rheology of the product due, it is theorized, to interaction between the components. This positive interaction is manifested in apparent viscosity increases (at shear rates up to 450 sec.sup.-1) and in yield point increase. The art details various kinds of structuring systems containing clay, polymer, or related combinations; but these are not completely satisfactory. The remarkable increase in yield point observed in this system together with superior hypochlorite stability further increases the desirability of the combination.
Improved rheology of the composition can result in improved detergent performance through better retention in the cup and better stability against separation which provides increased reliability in dosing the proper levels of active ingredients to the machine wash cycle Further the improved structuring system also results in improved product dispensability.